The present invention relates to a sensor system for determining an optical property, in particular the chlorophyl content or an illness or pest infestation, of a plant or a leaf by means of a reflection measurement, having a first light source, which emits light along a first beam axis, and a second light source, which emits light along a second beam axis, and at least one first receiver, which is aligned along a third beam axis, to detect light reflected from the plant.
Furthermore, the present invention relates to a corresponding method for determining an optical property of a plant, in particular the chlorophyl content or an illness or pest infestation.
Finally, the present invention relates to a method for analyzing a measurement to determine an optical property of a plant, in particular the chlorophyl content or an illness or pest infestation.
The fertilization of green plants using nitrogen is widespread in agriculture. During photosynthesis, nitrogen is used, inter alia, for the purpose of producing proteins. In this manner, the nitrogen promotes the growth of the plants. To allow the plants to absorb the nitrogen, fertilization is performed using an ammonium or nitrate salt. The farmer has to ensure an optimum nitrogen content of the plants, since both a lack of nitrogen and also an excess of nitrogen can impair the development of the plants. A lack of nitrogen is typically expressed in a lack of growth, a pale green color of the leaves, excessively early blooming, and yellowing of the plant. A nitrogen excess can result in lush growth, dark green leaves, delayed blooming, and spongy and soft leaf tissue. In addition, plants having nitrogen excess are frequently susceptible to frost and illness. For farmers, it is therefore desirable to be able to measure the nitrogen content of the plants in a field, in order to make a decision about the fertilization to be performed. In addition, the soil and the groundwater can thus also be prevented from being stressed with excessive fertilizer.
In particular, it is desirable in this case to be able to determine the nitrogen content in a contactless manner. Methods in which plant parts are clamped in measuring apparatuses or plants are pulverized and analyzed by means of chemical methods are excessively cumbersome and time-consuming for daily use and usage in the field. An example of this is found in the document US 2008 0239293 A1. Therefore, a demand exists in the market merely for contactless, in particular optical, measuring methods.
Nitrogen atoms do not display any absorption bands for optical radiation. The nitrogen content in the plant can therefore only be determined via an indirect route. Because of the close relationship between the nitrogen content of a plant and the chlorophyl content of a plant, the nitrogen content is determined indirectly via the chlorophyl content of the plant. The chlorophyl content of plants is strongly correlated with proteins of the photosystem, which contain the majority of the plant nitrogen, so that the nitrogen content of a plant can be inferred after a determination of the chlorophyl content.
Various optical methods are in turn known for determining the chlorophyl content.
A fluorescence measurement to determine the chlorophyl content is widespread. A sensor measures a short-term spontaneous emission of fluorescence radiation as a response to a flashing irradiation of the leaves. In this case, an irradiation device excites the molecules in the leaves in the photosynthetically active range using repeated laser flashes. This type of measurement is also called laser-induced fluorescence (LIF). The irradiation is performed using shortwave high-energy radiation, for example light in an orange wavelength range. The response of the chlorophyl-containing leaves is lower-energy radiation having somewhat greater wavelength than the excitation light, for example radiation in the near infrared range. Conclusions about the chlorophyl content of the plant can be drawn from the spectral composition of the fluorescent light. However, the fluorescence measurement has the disadvantage that the measurement can only be carried out on living plants. This is not critical in practice in the field; however, it has the result that, for example, a calibration of the sensors to detect the chlorophyl content must be carried out on living plants. Alternatively, for example, plates which are painted with a specific color cannot be used, wherein the color is to correspond to a specific chlorophyl content of a specific plant, in order to perform the calibration.
Furthermore, the fluorescence properties of plants change over the course of the day. Fluorescence measurements must therefore always be carried out in a predefined time window (for example, between 11 am and noon), so that the calibration and corresponding calculation algorithms apply. Examples of the determination of the chlorophyl content of a plant by means of fluorescence measurement are found, for example, in the documents EP 1 125 111 B1 and DE 60 306 675 T2.
Furthermore, reflection measurement to determine the chlorophyl content is known. A light beam having defined power is emitted onto the plant. The power of the fraction of the light beam reflected from the plant is measured and put in a ratio to the emitted power.
So-called passive optical reflection methods exist, in which daylight is used as the radiation source. However, the sun is a continuous light radiator, whose luminosity changes in a ratio of more than 100,000:1 during the course of the day. Therefore, sensors which use passive optical reflection methods in turn deliver measured values which are dependent on the position of the sun or the time of day, respectively.
Therefore, so-called active optical reflection methods have been developed, which use artificial optical sources, such as lamps, LEDs, or lasers, to irradiate plant parts. The radiation fraction reflected from the plants is analyzed in one or more wavelength ranges and compared to reference values. If green leaves are irradiated with light, they absorb to a large degree the blue light in a wavelength range of less than 480 nm and the red light in a wavelength range of 650 to 680 nm. As a result, the reflection of blue light and red light already saturates at low chlorophyl content. Light around 550 nm and radiation in a red to infrared range from approximately 700 to 730 nm, in contrast, are reflected more strongly, which has the result that they only saturate at a very high chlorophyl content. The higher the chlorophyl content, the lower the reflection at wavelengths around 550 nm and in the wavelength range from 700 to 730 nm.
The reflection values are strongly dependent on the angle between the normal of the leaf and the incidence direction. To determine the absolute chlorophyl content, a comparative value is therefore required, which is obtained from the reflection of red light. The optical power in the spectral range of the red light, i.e., in a wavelength range from 650 nm to 680 nm, is only reflected to a small degree from the plant, and the reflection of this reflected red light is only slightly dependent on the chlorophyl content of the leaf.
Therefore, at least two degrees of reflection are determined at different wavelengths to determine the chlorophyl content. Firstly, the degree of reflection is determined at a wavelength at which the reflection is strongly dependent on the chlorophyl content, for example in the case of green light. This chlorophyl-dependent degree of reflection is compared to a degree of reflection which was measured in a wavelength range which is substantially independent of the chlorophyl content of the plant. Both degrees of reflection are set in a ratio and form a so-called vegetation index, which can be calculated according to a plurality of previously proposed formulas. One example is the formula for the NDVI, in which
      N    ⁢                  ⁢    D    ⁢                  ⁢    V    ⁢                  ⁢    I    =                              ρ          grün                -                  ρ          rot                                      ρ          grün                +                  ρ          rot                      .  
A rising chlorophyl content in the leaves results in a reduction of the NDVI value and therefore in a decreasing scale value. In sensor systems which display this NDVI value to an operator, the operator must therefore continuously rethink, since not a higher, but rather a lower NDVI value means a better result. The NDVI values for strongly chlorophyl-containing leaves are at very small numeric values of approximately +0.2 and increase with decreasing chlorophyl fraction to +0.6.
The present invention is concerned exclusively with sensors which operate using an active optical reflection method.
A plurality of sensors for measuring the chlorophyl content are known on the market, which are either implemented as sufficiently small that they can be held in the hand by a person, or have larger dimensions, to be able to be used installed on a tractor, for example.
The measuring fields of the sensors are of different sizes and are established by the optical conditions. Sensors having smaller measuring fields have the disadvantage that they cannot be used alone for the rapid measurement of a larger area. A plurality of individual sensors is then to be used for larger areas, to be able to monitor a strip of a field which is multiple meters wide or the interior of a greenhouse, for example. Such assemblies rapidly become very costly due to the plurality of required sensors, however.
In the tractor-supported systems, for example, the YARA N sensor of Agri Con GmbH is known, which uses an actively measuring fluorescence method. A xenon flash lamp is located on each side of the sensor, which illuminates a strip of approximately 4 m width on each side of the tractor. A sensor sold under the name “MiniVeg N” is also known from Georg Fritzmeier GmbH & Co., which uses an active fluorescence method. A larger strip of field is also actively illuminated here, using a plurality of sensors in the case of the MiniVeg N.
In addition to the applied fluorescence method, these systems have the disadvantage that they must use very high-power light sources to illuminate the large measuring fields simultaneously and uniformly. These light sources not only represent a hazard for the eye safety of the user, i.e., the tractor operator, but rather require complex filter technologies to the emit light only in a non-visible range which still excites the plants, however, for example in the range of 700 nm. These sensors are therefore very costly.
The analysis of the measurement results is always performed via averaging. Since a relatively large area region is irradiated simultaneously, measured values of the radiation reflected from the plants and the radiation reflected from the ground are obtained inseparably and simultaneously during the analysis of the methods. In particular in early growth phases, in which the optimum fertilization state is very important, but the surface vegetation is only 10 to 20%, a relatively large area is therefore to be irradiated as diagonally as possible in the known systems, to be able to measure the most possible green leaf mass and least ground area for reliable averaging. However, young plants only cover the ground to a very small extent. They partially lie flat on the ground or only stand a few centimeters above the ground. Therefore, measuring devices which detect a large area perpendicularly or diagonally often only deliver poor results before the first and therefore most important administration of fertilizer.
Previous systems thus have disadvantages with respect to their dependence on daylight, the measurability of small plants, and the required high light powers. A demand therefore exists for an improved sensor assembly for determining the chlorophyl content of a plant, which remedies the above-described disadvantages.